A modern cell-phone communication network established and maintained by a cell-phone network service provider, conventionally referred to as an “operator”, typically provides full duplex, wireless communication services in a geographically localized service area between mobile communication devices. The devices operate under a contract with the operator, which permits their use of the network's services. The communication devices, referred to in the industry as mobile equipment (ME), appear in their most common format as the globally ubiquitous cell-phones, but may also appear as, or be comprised in, any of various other devices, such as a laptop, or personal digital assistant (PDA). For convenience of presentation, mobile equipment, ME, in any of its forms is generically referred to also as a cell-phone. An operator under contract to provide cell-phone communication services to a cell-phone is referred to as the cell-phone's, or cell-phone owner's, “home operator”.
Each cell-phone network is assigned a limited number of frequency channels over which to transmit and receive radio signals, and to provide for efficient use of the channels, the network's service area, is divided into contiguous communication regions referred to as cells. Each cell has a base transceiver station (e.g. BTS or Node-B) comprising antenna for transmitting and receiving radio signals to and from subscriber cell-phones and a transceiver for receiving and forwarding voice and/or data signals from and to subscriber cell-phones. The BTS also comprises a controller for processing and routing calls and performing network management tasks.
To reduce electromagnetic interference between communications in the network cells, adjacent cells in the network use different subsets of the network's assigned frequency channels, and subscriber phones and transceivers in the networks BTSs, normally transmit at relatively low power. As a result, a given BTS can efficiently transmit to, and receive signals from, cell-phones that are located only within a limited range from the BTS location. The range defines a size of the cell to which the BTS belongs. A cell-phone network service area may often cover a geographical area the size of a city and be divided up into tens of cells. In densely populated areas, BTSs in a cell-phone network may be spaced between 2 km-5 km (kilometers) apart, and between as little as a few dozen meters apart.
In order to provide continuous communication service for a subscriber cell-phone, the network's BTSs and the cell-phones communicate with each other over a set of control communication channels to continuously keep track of where in the service area the cell-phone is located. When the cell-phone “roams” from a first cell to a second cell in the network's service area, the network transfers the tasks of providing and maintaining communication services to the cell-phone from the BTS in the first cell to the BTS in the second cell. By appropriately coordinating transfer of responsibility for communication between cells, the cell-phone network is able to route calls substantially at any time to a given subscriber cell-phone anywhere in the network's service area, and maintain continuous communication to the cell-phone while the cell-phone physically moves from one network cell to another.
A subscriber's roaming is of course not limited to local, “home” roaming between cells within the geographically limited service area of a cell-phone network of the subscriber's home operator. Cell-phone network operators from different regions in a same country cooperate to provide “national roaming” from anywhere in the country and cell-phone network operators from different countries cooperate to provide global roaming. A subscriber of a given home operator may therefore typically use his or her cell-phone not only from a location within the service area of the home operator's network, but from substantially any location on the globe to communicate with a cell-phone anywhere else on the globe.
In order to provide satisfactory tracking of cell-phone locations, “hand-over” of communication responsibilities from cell to cell in a same cell-phone network, or between cells in different cell-phone networks, and billing for provided mobile telephony services, each cell-phone generally comprises a smart card, referred to as a subscriber identity module (generally referred to as a SIM or USIM). Hereinafter, a SIM or USIM may be referred to as a SIM or SIM card. The SIM is generally removably seated in a cradle in the phone, and is stored with a unique international mobile subscriber identity (IMSI) code that identifies the SIM. The SIM also comprises a typically 128 bit identity authentication key “KI”. The KI is generally not accessible from the SIM but is used by the SIM to generate responses to identity authentication challenges presented to the cell-phone by a cell-phone network.
In order to access services from a cell-phone network, the cell-phone must generally present its SIM card IMSI to the network. Following recognition of the IMSI, the cell-phone and network engage in an authentication procedure to authenticate the IMSI. In the procedure, the network challenges the cell-phone by sending it data, typically a random number, which the cell-phone's SIM processes using its KI to generate a response. The cell-phone transmits the response to the network, and if the response is what the network expects, it allows the cell-phone access to its services. In addition to circuit switched data (CSD) voice communication, the services may comprise, other CSD services e.g. Fax, signaling services e.g. short message services (SMS), and transmission and reception of packet switched (PS) data e.g. for Internet and Intranet access, directory access, file transfer and Instant messaging (IM).
Generally, once identified and authenticated by its home operator cell-phone network to which it is subscribed, barring aberrant events such as unforeseen disruption of services or security challenges, a cell-phone may roam its home network without further requirement of authentication. If however, the cell-phone roams from its home operator's network to a cell of a cell-phone network of another operator, it becomes a “visitor” to the other “visited operator”. The visited operator's network typically demands that the visitor identify itself by presenting its IMSI and authenticate its identity by generating a response to an authentication challenge. The visited operator transmits the visitor's IMSI and authentication challenge response to the visitor's home operator for validation. If validated, the visited operator allows the visitor to use its communication services.
Services provided to a cell-phone subscriber by cell-phone operators can be expensive and costs can rise substantially, and prohibitively, if a cell-phone roams to a visited operator and uses the services of the visited operator's cell-phone network—especially if the visited operator is a foreign operator. To control costs, a cell-phone user often purchases a plurality of different SIM cards, each associated with a different home operator and/or service package. When the user moves to different locations and/or uses different cell-phone services, the user chooses a SIM card intended to provide advantageous pricing for communication services that the user intends to use, and inserts it in to his or her cell-phone.